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First records of bentfin devil ray (Mobula thurstoni)
and the examination in physical factors of its
habitat in the western waters of Morotai Island
(North Moluccas)
To cite this article: D A Mukharror et al 2018 IOP Conf. Ser.: Earth Environ. Sci. 137 012048
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ASEAN-FEN INTERNATIONAL FISHERIES SYMPOSIUM – 2017
IOP Conf. Series: Earth and Environmental Science
137 (2018)
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‘’“” 012048
IOP Publishing
doi:10.1088/1755-1315/137/1/012048
First records of bentfin devil ray (Mobula thurstoni) and the
examination in physical factors of its habitat in the western
waters of Morotai Island (North Moluccas)
D A Mukharror1,4, I T Baiti2, S A Harahap3, D J Prihadi3, M Ichsan1, N Pridina1
1
Shark Diving Indonesia Morotai, Kampung Yayasan, Daruba, Morotai
Marine Science Student, FPIK, Universitas Padjadjaran, Jl. Raya Bandung-Sumedang KM 21,
Bandung Indonesia
3
Marine Science Department, FPIK, Universitas Padjdjaran, Jl. Raya Bandung-Sumedang KM
21, Bandung Iindonesia
2
E-mail : darmawan_ahmad_m@yahoo.com
Abstract. Bentfin devil ray (Mobula thurstoni) was recorded for the first time in Morotai waters
on 3rd March 2017. In a conservation management context, it is important to clarify the
population of Mobula thurstoni and their relations with their habitat. Thus, we examined the
existence of Mobula thurstoni with the physical parameters: depth, temperature, visibility,
current, weather, and tidal. We measured the existence of Mobula thurstoni with the Underwater
Visual Census (UVC) combined with Diver Operated Video (DOV) census. The research from
3rd March to 14th July 2017 (50 dives) showed the Frequency of Occurence (FO) is 50% per
single dive. The highest aggregation of 30 Mobula thurstoni was recorded at 14th May 2017 and
the average sighting was 3.1 Mobula thurstoni per single dive. Among the examined parameters,
it was found that strong factor affecting the sighting of Mobula thurstoni were at the depth of
30-35 m, temperature of 30C, visibility of 16-20 m, low current (< 1 knot), sunny weather, and
tidal category of B. Analysis of UVC and DOV results show that the research location was
categorised as feeding location for the Mobula thurstoni.
1. Introduction
Mobula rays and manta rays are categorized in mobulidae family and they are distributed globally in
both tropical and subtropical oceans. The mobulidae consists of two genera and 11 species [1] that are
harmless and feed mainly on zooplankton, shrimps, crabs, and small fishes [2, 3]. Recently, the manta
genera were redefined and they have at least two genus: the reef manta Manta birostris (Walbaum, 1792)
and the giant manta Manta alfredi (Krefft, 1868). The mobula genus comprises of nine recognized
species: the giant devil ray Mobula mobular (Bonnaterre, 1788), the Atlantic devil ray Mobula
hypostoma (Bancroft, 1831), the spinetail devil ray Mobula japanica (Mueller and Henle, 1841), the
shortfin devil ray Mobula kuhlii (Mueller and Henle, 1841), the Chilean devil ray Mobula tarapacana
(Philippi, 1893), the bentfin devil ray Mobula thurstoni (Lloyd, 1908), the pygmy devil ray Mobula
eregoodootenkee (Bleeker, 1959), the lesser Guinean devil ray Mobula rochebrunei (Vaillant, 1979),
and the Munk’s devil ray Mobula munkiana (Notarbartolo-di-Sciara, 1988).
Mobula thurstoni was known to occupy shallow neritic waters [4]. It has worldwide distribution with
the species reported from Indonesia [5, 6], the Arabian sea [7], Peru [8], Oman [9], the Philippines [10],
Chile [11], Mexico [12, 13], the Western Atlantic [4], the South China Sea [15], the Red Sea [16], the
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Gabon, Congo, Senegal and Angola [17], the Sao Tome e Principe [18], the Saint Peter and Saint Paul
Archipelago [19], the Australia, Mariana Islands, Ecuador, Maldives [20], the tropical Eastern Pacific
[21], India [22], and Sri Lanka [23].
Mobula thurstoni is classified as near threatened (NT) by the International Union for Conservation
of Nature and Natural Resources IUCN [4] with the exact population size is largely unknown. It is
estimated that of 9% of all mobulids (409) that were taken as bycatch in drift gillnets fishing for skipjack
tuna in the southern waters of Indonesia [5] is Mobula thurstoni. In the southern water of Indonesia,
approximately 155 Mobula thurstoni have landed at Tanjung Luar, Lombok Island and 109 others at
Cilacap, Central Java between 2001 and 2005 [24]. In Lamakera, East Nusa Tenggara, 200-300 Mobula
spp. were caught annually as they were harpooned during annual traditional harvesting [6]. 6there was
a decline for the Mobula thurstoni catch’s number data in 2013-2014 shows the 75% decline in Tanjung
Luar and 77% decline in Cilacap.
Mobula thurstoni is presumed to be long lived and slow growing according to Coutirier et al. [25]
with the number of pup of one per birth and disc wide of 650-850 at birth [13]. The largest size of
Mobula thurstoni was ever recorded was 1,870 mm disc width from Bohol Sea, the Philippines [26]. It
has estimated that Mobula thurstoni has age of maturity when its disc width (DW) is 1,538 mm [5]. In
all recorded specimens, female Mobula thurstoni were potentially having the larger size than of male
[3, 26, 5].
Bentfin devil ray in the Bohol Sea, the Philippines, feed almost exclusively on the specific krill family
of euphausiids: Euphausia diomedeae (89.66 %) with the trace of copepod, polychaete larvae, and plant
seed as the remaining diet [26]. In the Eastern Pacific, the examination of the stomach of Mobula
thurstoni indicated that this species is extremely specialized in its feeding habits, which comprise of
euphausiids species of Nyctiphanes simplex (86.7 %), Mysidium sp. (11.1 %), and the remaining is
copepod, megalopa larvae, hyperiids amphipods, fish eggs, Nemastocels diff., and stomatopods larvae
[3]. However, bentfin devil ray also has the capability to alter its diet depends on the availability of krill
by feeding predominantly on Euphausiids spp. (Nyctiphanes simplex) in the summer and Mysidium spp.
in the winter [27].
2. Methodology
Fifty research dives were conducted on one coral reef (research station) around the west waters of
Morotai Island in March – June 2017. Selection of the station is based on previous accidental sighting
of unknown Mobulid in 2013. The research station was located at N 02o 09’ 18.15” E 128o 12’ 24.43,
which is shown on the Morotai Island’s map (figure 1) below as Devil Ray Point.
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Figure 1. Research station in west waters of Morotai Island (source: Google Maps).
Materials used for this research were: SCUBA diving gear, underwater slate, transect meter,
squeezable empty mineral water bottle, GPS locator, and underwater camera. For analysis purposes, the
following tools were used: SPSSTM, Microsoft ExcelTM 2007, and Field Guide to the Identification of
Mobulid Rays. Survey was conducted at the research station regularly between February to June 2017.
Examination of the abundance of Mobula thurstoni was performed using Underwater Visual Census
(UVC) combined with Diver Operated Video (DOV) census. UVC has been used to estimate the
densities of the reef fishes since 1950s [28]. There were two methods of UVC: transect count (an
experienced diver swam at constant speed along the transect and noted the reef fish species observed on
a slate) and point count (an experienced diver stayed at a vantage point and noted the surrounding reef
fish species observed on a slate). UVC with transect and point counts have relatively similar level of
significance (p = 0.054 and 0.048 for 10 ha count respectively), therefore both census are deemed
reliable Watson et al. [29] to be selected for small survey area. A novel method, which is a combination
of UVC and diver operated video (DOV), presents great potential to count the abundance of reef fishes
in a research station [30].
The physical factors measured and recorded during the survey were: depth, temperature, visibility,
current, weather, and tides. As an addition, visual observation was performed to categorize the behavior
of the bentfin devil ray during research dives. Reef fish survey was performed in order to obtain the
abundance of cleaner fish in the research location. Identified reef fishes were then classified into their
cleaning habits based on classification on www.fishbase.org. The density of zooplankton in the research
location was also examined to observe the specific species of zooplankton existed that was related to
Mobula thurstoni.
3. Results
3.1 Mobula thurstoni existence
The first devil ray documented in Morotai was on March 3rd, 2017, as shown below in figure 2. Based
on the Field Guide on the Identification of Mobulid Rays Indo-West Pacific [31], Mobula thurstoni has
the following distinctive features: 1) ventral mouth; 2) white-tipped dorsal fin; 3) lack of spine at the
base of the tail; 4) long tail that equals with disc width in length when fully intact; 5) short necked
appearance; 6) short cephalic fins, length from the tip of each fin to the corner of mouth is less than 16%
of the total disc width; 7) white ventral marking does not extend above the eyes; 8) tail base that was
dorsoventrally compressed closed to the dorsal fin; 9) pectoral fin anterior margin that has a distinctive
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double curvature; 10) small sub-circular spiracle under the margin of the pectoral fin where it meets the
body; 11) coloration: general deep blue-back on dorsal surface; white ventral surface anteriorly, while
the distal half has a silver-brown sheen; 12)the top of the head has a dark band which stretches across
the head behind the eyes.
11
6
3
2
4
8
1
9
5
(a)
(b)
Figure 2. Appearance of Mobula thurstoni in research station (a) Ventral (b) dorsal.
3.2 Mobula thurstoni abundance
Research dives between March 3rd and July 14th, 2017 showed the average sighting of Mobula thurstoni
of 3.1 per dive with the maximum number of individual sighted in one dive was 30 Mobula thurstoni
on May 14th, 2017. The existence of Mobula thurstoni in every research dive in the research stations is
shown in figure 3, while table 1 shows the abundant indices of Mobula thurstoni. Figure 4 shows 6
probabilities of physical parameters that affected the sighting of Mobula Thurstoni and the dominant
sighting was at the depth of 30-35 m, temperature of 30 C, visibility of 16-20 m, low current (< 1 knot),
sunny weather, and tidal category of B.
Table 1. Abundant Indices of Mobula thurstoni.
Abundance Indices
Sighting (dive)
Number of dives
50
Total sighting
24
Total abundance of 1 individu
8
Total abundance of 2-10 individu
6
Total abundance of > 10 individu
8
Mean sighting frequency (%)
50
Number individuals per sighting
6.2
Number individuals per dive
3.1
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30
30
25
23
24
20
18
13
6/16/17
6/2/17
5/26/17
5/19/17
5
1
2
11 1
5/12/17
4/28/17
4/21/17
4/14/17
4/7/17
3/24/17
3/3/17
3/17/17
1
0
6
4
1
3
1
5/5/17
4
3/31/17
5
1
6/30/17
11
10
6/23/17
14
15
6/9/17
15
3/10/17
Number of Mobula thurstoni
(Individu)
35
Date of Sighting (Dive)
Figure 3. Existence of Mobula thurstoni in research stations.
We examined the existence of Mobula thurstoni with 6 physical parameters. Each parameters had 4
categories that are formed from the results. The categories of tidal in figure 4(f) shows that A is high to
low tide, B is low tide, C is low tide, and D is low to high tide. Figure 4 shows probability of Mobula
thurstoni sighting that was calculated from total sighting (24 dives) and each parameter’s categories.
(a)
(b)
(c)
(d)
(e)
(f)
Figure 4. Probability of Mobula thurstoni sighting on various physical
parameters (a) depth (b) temperature (c) visibility (d) current (e) weather (f)
tidal.
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3.3 Behavior of Bentfin Devil Ray
The research of Mobula thurstoni behavior in Morotai Island waters posed several states: feeding,
swimming, and cleaning. Pictures captured on their behavior are shown in figure 5 below, representing
all those three states mentioned above.
(a)
(b)
(c)
Figure 5. Behavior of Mobula thurstoni in research station (a) feeding (b) swimming (c) cleaning.
4. Discussion
4.1 Mobula thurstoni existence and abundance
Examination of the existence of mobula ray confirmed that the species of Mobula thurstoni existed in
the Western Waters of Morotai Islands. even though there was no real specimen collected from the
fishermen, the photographic physical identification resulted in 8 out of 12 of distinctive features of
Mobula thurstoni that were confirmed, as shown in the figure 2. No size measurement was conducted
during research survey due to the difficulty in getting near the Mobula thurstoni to obtain a minimum
parallax measurement error as they always got away when they were approached.
Despite the number of samplings that were far lower (50 dives compared to 4.222 dives), the
probability of sighting of Mobula thurstoni in Western Water of Morotai (50 %) was extremely higher
than the sighting likelihood in Eastern Pacific that was only 0.5 % [32]. The mean number of individuals
per sighting was 6.3 and sighting record of 30 individuals indicated that the research area has a potential
for large school of Mobula thurstoni for recreational diving attraction.
The dominant physical factors that affected the sighting of Mobula thurstoni were at the depth of 3035 m, temperature of 30 C, visibility of 16-20 m, low current (< 1 knot), sunny weather, and tidal
category of B.
Figure 6 shows R-square from 6 parameters: depth (0.14), temperature (0.75), visibility (0.1), current
(0.67), weather (0.9), and tidal (0). R-square is always between 0-1. If R-square is close to 1, then the
model fits your data. Temperature, current, and weather had R-square close to 1. Thus, the 3 parameters
had correlation to Mobula sighting at the research station.
Temperature can significantly influence poikilotherm physiological process, and this is likely a key
benefit to staying in warm shallow waters, despite the decreased opportunities for foraging [33]. As a
filter feeder, current helps Mobula thurstoni to catch more prey. The research was done during the
summer. it was predicted that in the summer, there was abundance of giant devil rays in the area
occupied by the Pelagos Sanctuary, the Sardinian Sea, and the Central Tyrrhenian Sea. Seasonal
movements may be related to possible energetic advantages deriving from warmer waters if this species
thermoregulates like other mobulids Alexander [2] or to the localized availability of high density of prey
Canese [35] e.g. mesopelagic and epipelagic fish in the Strait of Messina [36]. Research in other seasons
is recommended to know the abundance of Mobula thurstoni in each season
By having these dominant parameters, a potential underwater tourism of mobula ray sighting can be
developed in the future as an alternative to fisheries. To increase the quality of research, it is suggested
to explain scientifically why each factor contributes to the sightings.
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(a)
(b)
(c)
(d)
(e)
(f)
IOP Publishing
doi:10.1088/1755-1315/137/1/012048
Figure 6. Correlation and regression of Mobula thurstoni sighting and its physical parameters (a) depth
(b) temperature (c) visibility (d) current (e) weather (f) tidal.
4.2 Mobula thurstoni behavior
One of the early indications of the sighting was that the research location was considered as a feeding
station for Mobula thurstoni. Table 2 shows that the highest number of Mobula thurstoni sighting was
during feeding (14 dives). during feeding, mobulids were generally observed to swim forward with an
open mouth creating a passive water flow through the gill raker apparatus, a behavior referred to as ram
filter feeding [37, 38, 39]. Understanding the foraging ecology of threatened species will aid in their
conservation and management since feeding behavior often determines critical habitat use and spatial
patterns that are important in preventing or mitigating targeted or incidental capture and other human
impacts [40].
The cleaning behavior of Mobula spp. has never been documented prior to 2016 Murie et al. [25] and
none of the research on the Mobula thurstoni cleaning has been conducted. Interaction between Mobula
kuhlii and the blue streak cleaner wrasse, Labroides dimidiatus, were observed in Mozambique [25].
The difference between Mobula kuhlii cleaning behavior in Mozambique and the Mobula thurstoni
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behavior in Morotai was the latter’s cleaning activity can be conducted even with only single individual
while the previous one cited that the Mobula kuhlii preferred to aggregate (more than one individual)
while cleaning [41].
Table 2. Behavior of Mobula thurstoni in research station.
Behavior
Feeding
Swimming
Cleaning
Sighting (dive)
14
6
4
4.3. Mobula thurstoni and zooplankton density
Morotai Island Waters lies in the tip of NECC (North Equator Counter Current) area and classified as
very rich area of zooplankton [42]. Previous marine survey showed that the research location had
zooplankton’s density of 0.757 – 1.070 mg/m3 and categorized as the highest (approximately 600
individuals per m3) in Morotai Waters [43]. Mobula thurstoni study in the Philippines revealed that the
zooplankton species of Euphausiids was their dominant preys [26].
5. Conclusion
The existence of Mobula thurstoni (Lloyd 1908) was confirmed through the identification of the
photographs and videos. Among the examined parameters, it was found that factors that strongly
affecting the sighting of Mobula thurstoni were at depth (30-35 m), temperature (30 C), visibility (1620 m), low current (< 1 knot), sunny weather, and tidal category of B. Early indication of research site
categorized as feeding station was supported by the fact the most sighting was observed during feeding
behavior.
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Acknowledgements
We thank the research team in Morotai Island for their support during data collection as well as to
Marine Science Department of Universitas Padjadjaran for offering many suggestions and comments
on the manuscript.
9